1. Field of the Invention
The present invention relates to a method of manufacturing a high-strength aluminum alloy extruded product excelling in corrosion resistance and stress corrosion cracking resistance. More particularly, the present invention relates to a method of manufacturing a high-strength aluminum alloy extruded product excelling in corrosion resistance and stress corrosion cracking resistance which is suitable in application as structural materials for transportation equipment such as automobiles, railroad carriages, and aircrafts.
2. Description of Background Art
In recent years, emission regulations have been tightened from the viewpoint of protection of the global environment. In the field of manufacture of structural members and components for transportation equipment such as automobiles, the reduction of vehicle weight has been vigorously pursued to save fuel consumption and hence to decrease the emission of carbon dioxide and other noxious gases. An effective means to reduce the vehicle weight is the use of aluminous materials instead of conventionally used ferrous materials.
The 6000 series (Al—Mg—Si) aluminum alloys as represented by an AA6061 alloy and AA6063 alloy are widely employed in practical applications in transportation equipment components due to excellent workability, easiness of production, and excellent corrosion resistance. However, since the 6000 series alloys have disadvantages in strength in comparison with high-strength aluminum alloys such as the 7000 series (Al—Zn—Mg) alloys and the 2000 series (Al—Cu) alloys, an increase in the strength of the 6000 series aluminum alloys has been attempted. For example, an AA6013 alloy, AA6056 alloy, AA6082 alloy, and the like have been developed.
These alloys possess improved strength in comparison with the conventional AA6061 alloy or the like. However, further progress in the reduction of the vehicle weight is making requirements for thinner and lighter materials even more demanding. Since there still have been cases where the above alloys are not wholly satisfactory in terms of strength, corrosion resistance, and stress corrosion cracking resistance, there is proposed an aluminum alloy comprising 0.5 to 1.5% of Si, 0.9 to 1.5% of Mg, 1.2 to 2.4% of Cu, wherein the composition of Si, Mg, and Cu satisfies the conditional equations 3≦Si %+Mn %+Cu %≦4, Mg≦1.7×Si %, and Cu %/2≦Mg %≦(Cu %/2)+0.6, and further comprising 0.2 to 0.4% of Cr, while limiting Mn as an impurity at 0.05% or less, with the balance being Al and unavoidable impurities (Japanese Patent Application Laid-open No. 8-269608).
However, this aluminum alloy is mainly used as a sheet material and has the disadvantage of inferior extrudability and inferior characteristics of extrusions in extrusion application, particularly when extruded into a hollow profile by using a porthole die or a spider die. In order to overcome this problem, one of the inventors of the present invention, together with other inventors, reviewed the above composition and proposed an Al—Cu—Mg—Si alloy extruded product for application in structural members of transportation equipment (Japanese Patent Application Laid-open No. 10-306338). This aluminum alloy extruded product is excellent in extrudability into a hollow profile and is characterized in that, when a tensile test is conducted for the weld joints inside the extruded hollow cross section by applying a tensile stress in the direction perpendicular to the extrusion direction, the aluminum alloy extruded product fractures at locations other than the weld joints.
However, if the above aluminum alloy extruded product is used in a reduced thickness, the aluminum alloy extruded product is not entirely capable of providing the required strength. In order to improve the characteristics of the above Al—Cu—Mg—Si alloy extruded product, one of the inventors of the present invention together with other inventors further proposed to add Mn to the Al—Cu—Mg—Si alloy and to control the thickness of the crystal layer of the Al—Cu—Mg—Si alloy extruded product, thereby providing a high-strength alloy extruded product having excellent corrosion resistance (Japanese Patent Application Laid-open No. 2001-11559). However, this aluminum alloy exhibits poor extrudability in comparison with conventional alloys such as the AA6063 alloy due to high deformation resistance. In particular, when successive billets are supplemented for a continuous extrusion of a solid product, it is necessary to provide a flow guide at the front of the solid die. However, this aluminum alloy suffers from deficiencies such as extrusion cracking occurring at the corners of the extruded product and a tendency for forming a coarse surface grain structure, thereby causing a deterioration in strength as well as in stress corrosion cracking resistance.
Moreover, in the case where a hollow product is extruded by using a porthole die or a bridge die, this aluminum alloy also presents problems such as extrusion cracking and a tendency for forming a coarse grain structure along the joints, thereby causing a deterioration in strength, corrosion resistance, and stress corrosion cracking resistance.
The present invention has been achieved after extensive experiments and investigations conducted in an attempt to solve the above-described problems associated with high-strength aluminum alloy extruded products, including studies concerning the relationship between the characteristics of the extruded product and dimensions of the die as well as various parts of flow guides, applicable when a solid product is extruded using a solid die alone or using a solid die together with a flow guide attached thereto, and studies concerning the relationship between the characteristics of the extruded product and the difference in flow speeds of the aluminum alloy inside the extrusion die, applicable when a hollow product is extruded by using a porthole die or a bridge die. Accordingly, an object of the present invention is to provide a method of manufacturing an aluminum alloy extruded product excelling in corrosion resistance, stress corrosion cracking resistance, and strength, as achieved by effectively preventing the occurrence of extrusion cracking or formation of a coarse grain structure in the extruded product.